1. Field of the Invention
The invention relates to a method of detecting and evaluating analog photometric signals in a test carrier analysis system, wherein the test field of a test carrier is irradiated by a light source clocked in light-dark phases, the light reflected by the test field is detected by a measurement receiver and its measured value is passed to a measurement integration and digitalization circuit for evaluation, and wherein, over a measurement period comprising several light and dark phases of the clocked light source, the measured value is integrated over at least part of each light phase and over at least part of each dark phase, and a total integrated value is formed from the integrals of the light and dark phases in the measurement integration and digitalization circuit.
The invention further relates to an arrangement for carrying out the method.
2. Description of the Related Art
Test carrier analysis systems are increasingly used for the analysis of fluids, especially body fluids, such as blood or urine, for the purpose of medical examination. These systems consist essentially of evaluation apparatuses and the appropriately designed test carriers.
The test carriers frequently take the form of test strips whose test fields are conventionally made up of a base layer and at least one test layer arranged thereon. The test layer contains one or more reagents. When a sample, for example a body fluid, is applied to the test layer, a chemical reaction takes place with components of the sample and leads to a detectable change in the test field, especially to a color change in a detection layer. This change can be evaluated for quantitative analysis by means of suitable methods and corresponding apparatuses.
Thus, for example, after the reaction has taken place, a reflection-photometric measurement by means of the evaluation apparatus can be used to infer, from the diffuse reflectivity of the test field, the concentration of that component of a body fluid which is to be determined. In other cases, the desired analysis result can be derived from the change in reflectivity over time.
Particularly stringent accuracy demands are made on the detection and evaluation of the photometric signals in order to detect the very small measurement currents or measurement voltages with sufficient resolution for determination of the generally small change in reflectivity and to permit quantitative analysis. On the other hand there is a large number of sources of interference. These are on the one hand the known problems associated with evaluation electronics for the detection of very small signals, such as amplifier drift, superimposed direct voltages, high-frequency noise voltages, low-frequency alternating and ripple voltages, due especially to pick-ups at mains frequency, amplifier input currents and creeping or residual currents of various kinds.
In addition there are the typical interferences associated with optical measurements, caused by secondary and stray light in the form of disturbing external light. These include both relatively constant external light components and typical interferences from mains-operated light sources working at the conventional mains frequency of 50 Hz or 60 Hz.
Hitherto, test carrier evaluation apparatuses have normally been constructed so that, to avoid the abovementioned disturbing external light components, external light has been prevented as far as possible from entering the measurement area of the apparatus, where the test field, the clocked light source and the measurement receiver are located during the measurement. For this purpose, the test carrier was inserted, for example, in a narrow channel. In other apparatuses, the test carrier holder for accommodating the test carrier during the measurement was located inside a lightproof housing with a flap, which had to be opened for insertion of the test strip and closed for the measurement.
These known light-shielding measures have disadvantages associated with them. Narrow channels easily become soiled when the test carrier, for example a blood sample, is inserted. In the second case, the construction and manufacturing costs are increased by the need for a flap. Moreover, it has been found that the flap is frequently a cause of operating errors if it is not completely closed or is opened too early. It has to be taken into consideration here that test strip evaluation apparatuses are used principally by diabetics for self-determination of their glucose levels. However, the nature of their illness is such that diabetics often have limited manual dexterity or impaired vision.
For larger analysis apparatuses, including in some cases those which form part of a test carrier analysis system, proposals have already been made for the additional electronic suppression of the interferences caused by disturbing external light. Here the light source is clocked in light and dark phases. The signals from the measurement receiver are then evaluated over part of both the light phase and the dark phase, after which subtraction of the result of the dark phase from that of the light phase makes it possible substantially to eliminate interferences which make additive contributions to the measurement signal and change slowly in relation to the clock frequency of the light source.
Suitable methods of suppressing high-frequency interferences are those in which the signal from the measurement receiver is integrated over a certain measurement period. This substantially suppresses interferences whose frequencies are much greater than the reciprocal integration time.
German patent specification 28 14 358 discloses a method and a corresponding arrangement which combine these two measures. Here a measurement receiver is irradiated by the clocked light source and the signals are integrated over part of both the light phase and the dark phase. In this known method, the signal from the measurement receiver is evaluated over both the light phase and the dark phase and the two digitalized results are then subtracted from one another to give a corrected measured value.
European patent specification 0 075 767 proposes a method and a corresponding arrangement in which measuring errors of the abovementioned type are suppressed in a simple manner and with low construction costs, the resolution and accuracy being high. Said patent specification discloses a method of the type indicated at the outset, where integration over several consecutive light and dark phases is carried out without intermediate digitalization, thereby realizing a long integration time at a nevertheless relatively high clock frequency of the light source. This largely eliminates on the one hand high-frequency interferences which are still of relatively low frequency because of the long integration time, and on the other hand direct voltage components which also change relatively rapidly because of the relatively high clock frequency.